1. Field of the Invention
The present invention relates to the field of sublithographic fabrication of electronic circuits, in particular molecular electronics. More specifically, an array-based architecture is disclosed using a collection of techniques where small feature sizes are realized without lithographic processing. The arrays may be configured post-fabrication to implement any computable function of some finite size determined by the size of the arrays.
2. Description of the Prior Art
Today carbon nanotubes which are nanometers in diameter and microns long can be synthesized. See, for example, Cees Dekker, Carbon nanotubes as molecular quantum wires, Physics Today, pp. 22–28, May 1999. The growth and alignment of these nanotubes can be controlled, such that the nanotubes can be assembled into parallel rows of conductors and layered into arrays. See, for example, Yu Huang, Xiangfeng Duan, Qingqiao Wei, and Charles M. Lieber, Directed assembly of one-dimensional nanostructures into functional networks. Ultimately, the nanotubes can be a single nanometer wide and spaced several nanometers apart.
At the same time, technologies to grow silicon nanowires are being developed. See, for example, Yi Cui, Lincoln J. Lauhon, Mark S. Gudiksen, Jianfang Wang, and Charles M. Lieber, Diameter-controlled synthesis of single crystal silicon nanowires, Applied Physics Letters, 78(15):2214–2216, 2001, and Alfredo M. Morales and Charles M. Lieber, A laser ablation method for synthesis of crystalline semiconductor nanowires, Science, 279:208–211, 1998. Also the silicon nanowires are only nanometers in width and can be grown or assembled into sets of long parallel wires. See, for example, Yong Chen, Douglas A. Ohlberg, Gilberto Medeiros-Ribeiro, Y. Austin Chang, and R. Stanley Williams, Self-assembled growth of epitaxial erbium disilicide nanowires on silicon(001), Applied Physics Letters, 76(26):4004–4006, 2000. The electrical properties of these silicon nanowires can be controlled with dopants, yielding semiconductor wires, as shown, for example, in Yi Cui, Xiangfeng Duan, Jiangtao Hu, and Charles M. Lieber, Doping and electrical transport in silicon nanowires, Journal of Physical Chemistry B, 104(22):5213 5216, Jun. 8, 2000.
It is also known how to build nanoscale crosspoints. FIG. 1 is a schematic cross-sectional view which shows a suspended prior art nanotube conductor 1 coupled to a plurality of lower carbon nanotube or silicon nanowire conductors 2, 3, and 4 through a plurality of supports 5. The supports are made of a dielectric material, such as silicon dioxide. In this way, a nanotube-nanotube (or nanotube-nanowire) junction is formed. The junction is bistable with an energy barrier between the two states. In one state, see tubes 1–2 and 1–4, the tubes are “far” apart and mechanical forces keep the top wire 1 from descending to the lower wire 2, 4. At this distance the tunneling current between the crossed conductors is small, resulting, effectively, in a very high resistance (GigaOhms) between the conductors. In the second state, see tubes 1–3, the tubes come into contact and are held together via molecular forces. In this state, there is little resistance (about 100 KΩ) between the tubes. Therefore, by applying a voltage to the tubes, one can charge them to the same or opposite polarities and use electrical charge attraction/repulsion to cross the energy gap of the junction between the two bi-stable states, effectively setting or resetting the programming of the connection. These junctions can be rectifying such that the connected state exhibits PN-diode rectification behavior. Molecular electronics PN-junctions are disclosed, for example, in Y. Cui and C. M. Lieber, “Functional Nanoscale Electronic Devices Assembled using Silicon Nanowire Building Blocks,” Science 291, 891–893 (2001).
Also known in the prior art is how doped silicon nanowires can exhibit Field-Effect Transistor (FET) behavior. FIG. 2 is a schematic perspective view of a prior art embodiment which shows oxide 10 grown over a silicon nanowire 11 to prevent direct electrical contact of a crossed conductor 12, for example a carbon nanotube or a silicon nanowire. The electrical field of one wire can then be used to “gate” the other wire, locally evacuating a region of the doped silicon nanowire of carriers to prevent conduction. FET resistance varies from Ohms to GigaOhms. Similarly, also carbon nanotubes can exhibit FET behavior. See, for example, Yu Huang, Xiangfeng Duan, Yi Cui, Lincoln Lauhon, Kevin Kim and Charles M. Lieber, “Logic Gates and Computation from Assembled Nanowire Building Blocks,” Science, 2001, v294, p 1313–1317, V. Derycke, R. Martel, J. Appenzeller and Ph. Avouris, “Carbon Nanotube Inter- and Intramolecular Logic Gates,” Nano Letters, 2001, v1n9, p 435–456, and Sander J. Trans, Alwin R. M. Verschueren and Cees Dekker, “Room-temperature Transistor Based on a Single Carbon Nanotube,” Nature, 1998, v393, p 49–51, May 7.
Furthermore, regular arrangements of nanoscale wires (parallel arrays of wires, crossed, orthogonal structures) are also known. A crossbar is usually defined as an array of switches that connect each wire in one set of parallel wires to every member of a second set of parallel wires that intersects the first set. Generally, the two sets of wires are perpendicular to each other. An interesting consequence of all these devices is the ability to store state and implement switching at a wire crossing. That is, the switch device itself holds its state. Therefore, crossbars in this technology can be fully populated with no cost in density. This is particularly beneficial in achieving the necessary defect tolerance. See, for example, U.S. Pat. No. 6,256,767 to Kuekes and Williams.
The prior art also discloses how to build a wide range of electronic circuits where features at the scale of the device features (e.g. VLSI) can precisely be placed. Additionally, techniques for building universally programmable devices (e.g. PALs, PLAs, connections thereof) having VLSI fabrication capabilities are also known.
Recently, it is also known how to build small collections of non-restoring molecular scale logic and how to connect together non-restoring molecular scale logic at the microscale. See, for example, C. P. Collier, E. M. Wong, M. Belohradsky, F. M. Raymo, J. F. Stoddard, P. J. Kuekes, R. S. Williams, and J. R. Heath, “Electronically configurable molecular-based logic gates,” Science, vol. 285, pp. 391–394, 1999.
Also known is an architecture based on molecular-scale electronic building blocks, called ‘nanoFabrics.’ See Seth Copen Goldstein and Mihai Budiu, “Nanofabrics: Spatial computing using molecular electronics,” in Proceedings of the 28th Annual International Symposium on Computer Architecture, June 2001, pp. 178–189. However, the architecture disclosed in Goldstein is restricted to the use of two-terminal devices only and does not teach how nanoBlocks are customized.
It is still not known how to connect together large numbers of these nanoscale or sublithographic devices to create arbitrary logic functions. Additionally, it is still not known how to arrange for arbitrary connection of (cascading of) logic circuits at the nanoscale level without need for returning to a micro-scale level for signal restoration. It is also not known how to exploit the limited assembly techniques now possible to build arbitrary logic functions. It is also not known which logic structures are efficient when dealing with the cost constraints imposed by these fabrication techniques.
Throughout the present disclosure, the term micron-scale will refer to dimensions that range from about 0.1 micrometer to about 2 micrometers in size. The term nanometer-scale (also nanoscale) will refer to dimensions that range from 0.1 nanometers to 50 nanometers (0.05 micrometer), the preferred range being from 0.5 nanometers to 5 nanometers.